The present invention relates to an interference cancellation system which cancels an undesired interference signal in a radio communication receiver.
A principle system for cancelling an interference signal is shown in FIG. 17. In FIG. 17, the reference numeral 201 is a main antenna which receives both the desired signal and the undesired interference signal, 202 is an auxiliary antenna which receives only an undesired interference signal, 203 is an amplitude-phase control circuit, 204 is a combiner, 205 is a correlation detector, 207 and 208 are frequency converters each of which includes an amplifier, 209 is a low-pass filter, and 206 is an output terminal which provides the output signal which is free from an interference signal. The main antenna 201 is directed to a desired signal, but receives both the desired signal and the undesired interference signal. The auxiliary antenna 202 is directed to the undesired interference signal, and receives only the interference signal. The interference signal received by the auxiliary antenna 202 is applied to the amplitude-phase control circuit 203 which adjusts the amplitude and the phase of the interference signal so that the output of the control circuit 203 has the equal amplitude and the anti-phase as that of the interference signal received by the main antenna 201. Thus, when the combiner 204 combines the interference signal from the main antenna with the output of the amplitude-phase control circuit 203, the interference signal is cancelled, and the desired signal is obtained at the output of the combiner. The control signal for controlling the amplitude-phase control circuit 203 is obtained by the correlation detector 205, which provides the correlation between the undesired signal received by the main antenna 201 and the undesired interference signal received by the auxiliary antenna 202.
However, when the apparatus of FIG. 17 is implemented, a correlation circuit, and a control circuit must satisfy many requirements, which are not easy. Therefore, the mere principle circuit of FIG. 17 is not practical.
The practical circuit in a prior art is shown in the U.S. Pat. No. 4,384,366 (the corresponding West German Pat. No. P31 10 602, the UK Pat. No. 2072995, the French No. 8106258, Canadian Pat. No. 1,167,158). In that circuit, a single frequency converter is used, and an interference signal is applied to an amplitude-phase control circuit which provides the compensation signal having the equal amplitude and the anti-phase to a combiner in a main signal path. In order to adjust amplitude and phase of a control signal in the amplitude-phase control circuit, the control signal is subject to perturbation by low frequency signal. The residual interference component is fluctuated in accordance with the low frequency signal. The residual interference component is detected by an envelope detector. And the correlation is taken between a residual interference component and the low frequency signal. The control is effected so that the correlation is the smallest.
However, the technique has the disadvantage that a circuit structure is complicated, because of the presence of a perturbation circuit, and an additional envelope detector.